What is the model number of the Motorola router and its current firmware version? Might be able to install a firmware update to fix the router which may well be the root cause of the problem. But it is possible there is no update to add the necessary improvement required by newer operating systems.
Sounds like an older router that uses SMBv1 instead of newer SMBv2 or SMBv3. What that means is that the Motorola router is serving files over your network from the USB drive using an old file sharing protocol. SMB = Small Message Block and it is the underlying Windows file sharing protocol. 3rd party appliances such as the Motorola router and many brands and products typically use the open source SAMBA project to share files with Windows / Mac / Linux. The version 1.0 protocol is ancient while version 2.0 and 3.0 are much newer. Microsoft and Apple have mostly dropped support for the old SMB 1.0 protocol.
10.13.6 is High Sierra and I think SMBv1 was turned off by default since Catalina 10.15.x.
Two options:
- Replace your router to a modern one that offers a USB port from which you can share a printer or USB drive using SMBv2/3
- Keep the old router but replace the USB drive attached to the router with a small 4-5 port network switch and a mini-NAS (networked array storage) appliance which offer two or more disks and can act as a Time Machine drive, providing network shares at the same time time and can even run server software. A mini-NAS would connect to your network via Ethernet using that small switch and serve the files from the NAS instead of from the USB drive connected to your router. The switch would go between the router and the NAS giving you extra Ethernet ports.
3rd option (not recommended):
- Enable SMBv1 on the new Mac which is in fact, somewhat of a security risk. Remember the news about Edward Snowden leaking NSA secrets and fleeing to Russia? Well he revealed that the NSA could hack any system using SMBv1 along with every smartphone on the planet. Microsoft fixed it and then even updated Windows XP despite it being officially end of life because so many Windows XP machines were still running online. Microsoft typically disables SMBv1 and most IT security people would as well in a corporate network. Apple defaults to SMBv2 or SMBv3 since Catalina and SMBv1 is turned off.
You can see this post that covers how to enable SMBv1 but I wouldn't recommend it except as an exercise to prove the Motorola router is using SMBv1 protocol. Can I active or enable SMBv1 in Catalina? - Apple Community
You can try it and see if the problem goes away, but you really should turn SMBv1 off and upgrade your router or your storage solution with a NAS. Synology would be the brand of NAS I would recommend over all the others. I would avoid QNAP as they are notorious for serious firmware issues.
